The present invention relates to a circuit for detecting the presence of a human body.
In particular, the present invention is advantageously, but not exclusively, applicable to the field of motor vehicles to enable a user to control automatic opening of the door locks of a vehicle in xe2x80x9chands-freexe2x80x9d manner and without making use of a remote control that needs to be actuated by the user, and to do so specifically when the user comes close to the vehicle, and in particular when the use takes hold of the outside handle of a door.
At present, various solutions are known for detecting the presence of a person""s hand. The best known solutions are based on capacitive detection, specifically on detecting a variation in the capacitance of a capacitor due to the finger of the person coming flush therewith, while others are based on proximity detection principles, e.g. making use of the Doppler effect, of light beams being interrupted, of heat being detected, etc.
Unfortunately, known solutions require circuits to be implemented that are complex and expensive and that are found to be difficult to provide in motor vehicles.
The object of the present invention is to provide a presence detection circuit which is simple and economical.
To this end, the invention proposes a circuit for detecting presence, the circuit being characterized in that it comprises:
receiver means comprising an antenna-forming element;
oscillator means receiving as input the output signal from said receiver means, the oscillator means including a resonant stage and an amplification stage connected to the input of said resonant stage and providing said oscillator means with gain such that the frequency of oscillation of the signal output by the oscillator means is modified when at least a portion of a person, e.g. the hand, is in the immediate proximity of the antenna-forming element of the receiver means; and
processor means for detecting a variation in the frequency of oscillation of the signal output by the oscillator means and for generating an information signal representing the detection of a presence when such a frequency variation is detected.
This circuit is advantageously associated with the various following characteristics taken individually or in any technically feasible combination:
the amplification stage provides the oscillator with gain of the order of or greater than 105, at least for a frequency lying in the range 100 Hz to 500 Hz;
an input of the amplifier stage is connected to the output of the receiver means via first resistor means, and the amplifier stage has at least one branch which is connected in parallel with said resistor means and which comprises at least three logic inverters in cascade;
the logic inverters are amplifiers constituting inverting logic gates having push-pull type output circuits;
that second resistor means are connected between the output of the receiver means and the three logic inverters in cascade;
the resistances of the first resistor means and of the second resistor means are such that at least the first logic inverter operates in non-saturated mode;
the non-saturated logic inverter(s) is/are followed by at least one pair of saturated logic inverters;
resistor means are connected in parallel with the terminals of each of the logic inverters in cascade;
the processor means include comparator means for comparing a predetermined threshold value with the mean value of a signal representative of variation in the frequency of the signal output by the amplifier stage;
the resonant stage comprises logic inverter means connected between the input and the output of said oscillator means, and capacitor and resistor means connected in series between the output of said receiver means and the output of the amplifier stage;
the processor means comprise integrator means connected to the output of the amplifier stage;
said integrator means comprise logic inverter means having an input connected to the input of said integrator means via unipolar switch means, and capacitor means connected between the input of said logic inverter means and a ground line;
said integrator means further comprise variable resistor means connected between the input of said fifth logic inverter means and said ground line;
the comparator means comprise logic inverter means having an input connected to the input of said comparator means via unipolar switch means, capacitor means connected between a power supply line and the input of said logic inverter means, and resistor means connected in series with the unipolar switch means between the input of said logic inverter means and the ground line;
the comparator means include an adaptive threshold comparator circuit; and
the receiver means include means for filtering disturbances due to capacitive effects.
The invention also provides a circuit which comprises:
means for receiving an intrinsic electromagnetic field radiated by the human body;
means for amplifying and converting the electromagnetic field received by said receiver means, generating a signal having at least one parameter which is correlated with the intensity of said received electromagnetic field; and
comparator means receiving as input said signal and comparing said parameter with a threshold value representative of presence being detected.
The invention also provides a system for unlocking vehicle doors, in particular car doors, the system being of the xe2x80x9chands-freexe2x80x9d type and including a presence detection circuit and means for emitting an interrogation signal which are woken up when the detection circuit detects the presence of a person, said detection circuit being a presence detection circuit of the above type.